IMPLICATIONS OF DIFFERENT STRING
LENGTHS FOR AN MPP TRACKER
Relevance of mismatch losses and effects on the total energy yield for
commercial photovoltaic systems
.........................................................................................................................................................................
SE_WP_Implications_of_different_string_lengths_EN.docx
2/19
© Fronius International GmbH
Version V1.0 11/2021
Solar Energy
Authors: Peter Schmidhuber, Jasmin Gross
Fronius reserves all rights, in particular rights of reproduction, distribution and translation. No part of this document may be reproduced,
in any form whatsoever, or stored, processed, duplicated or disseminated with the aid of electronic systems, without the written consent
of Fronius. You are hereby reminded that the information published in this document, despite the greatest care being exercised in its
preparation, is subject to change and that neither the author nor Fronius can accept any legal liability. Gender-specific wording refers
equally to the male and female form.
.........................................................................................................................................................................
SE_WP_Implications_of_different_string_lengths_EN.docx
3/19
TABLE OF CONTENTS
1 Introduction ..................................................................................................................................... 4
2 Occurrence of mismatch losses ................................................................................................... 5
3 Simulation........................................................................................................................................ 7
3.1 Ideal situation example: 14 strings each with 21 PV modules ......................................................... 7
3.2 13 strings each with 21 PV modules | 1 string with 22 PV modules ................................................. 8
3.3 5 strings each with 21 PV modules | 9 strings each with 22 PV modules ...................................... 10
3.4 1 string with 21 PV modules | 13 strings each with 22 PV modules ............................................... 11
3.5 Overview ......................................................................................................................................... 12
4 Conclusion .................................................................................................................................... 13
5 Appendix........................................................................................................................................ 15
6 List of figures ................................................................................................................................ 19
7 List of tables .................................................................................................................................. 19
.........................................................................................................................................................................
SE_WP_Implications_of_different_string_lengths_EN.docx
4/19
1 INTRODUCTION
It is important to pay attention to general design rules in PV module string design and the planning of PV
systems. However, some design rules state that only an equal number of PV modules should be
connected to an MPP tracker so that losses can be excluded. Due to advances in modern technology, this
guideline no longer applies in some cases.
Local requirements and individual conditions mean that a completely symmetrical string arrangement on
roofs is sometimes not possible. As a result, the roof surface often does not permit an equal number of PV
modules from strings connected in parallel. In the case of inverters with an MPP tracker, according to the
aforementioned design rule, this would mean that the entire string would have to be omitted. However,
omitting an entire string can lead to a loss of several kWp in the commercial sector.
It is not only spatial conditions that often hinder symmetrical string arrangement. Due to local conditions or
visual expectations, a very specific number of PV modules sometimes need to be implemented.
In this document, the actual effects of a string configuration with unequal lengths is shown. Using the
PV*SOL configuration tool, the effects of PV module strings connected in parallel with different lengths are
analyzed.
Contrary to the general presumption that “mismatch losses” minimize the total energy yield of a PV
system, it can be shown based on calculations that varying string lengths are certainly possible and also
useful, as only the additional PV modules would experience any yield losses. However, ultimately the
additional PV modules would deliver a higher yield for the installation overall.
.........................................................................................................................................................................
SE_WP_Implications_of_different_string_lengths_EN.docx
5/19
2 OCCURRENCE OF MISMATCH LOSSES
As the name suggests, the Maximum Power Point Tracker (MPPT) in an inverter attempts to continuously
find the operating point with the maximum output of the connected PV generator and operate it at this
point. If the generator is permanently operated at its MPP, it generates maximum yield.
A PV generator often consists of more than one PV module string. The PV module strings can also have
various MPPs due to different alignment, inclination, and number of PV modules. In the case of different
MPPs, the MPP tracker attempts to adjust the PV generator to the operating point that delivers the
maximum output for all PV modules and strings connected to a tracker. As not every string can be
operated at its individual optimum operating point, this can lead to energy losses compared to theoretically
recoverable energy. These losses are called mismatch losses.
In general, every PV system exhibits low mismatch losses, as all conditions for two completely identical
PV strings can never prevail (for example, the PV modules themselves always differ slightly).
The term mismatch losses is used frequently in the context of different string configurations. Situations in
which a deviation in the strings is created deliberately within a PV generator - either through different
alignment of the PV modules or a different number of PV modules in the string.
The following figure shows an example of an installation where one string out of a total of 14 strings has
intentionally been expanded by a single PV module. In comparison to the other 13 strings, this string
consists of 22 instead of 21 PV modules. However, the 14 strings are all connected to one MPPT.
Figure 1: One of a total of 14 strings has 22 instead of 21 PV modules
.........................................................................................................................................................................
SE_WP_Implications_of_different_string_lengths_EN.docx
6/19
In the graph below, it can be seen that at full irradiation, the one longer string with 22 PV modules (red
curve), has a somewhat higher voltage (904 V) at MPP than the shorter strings (blue curve, 864 V).
If all strings are considered together, this produces the green line. The green line is the curve on which the
MPP tracker of an inverter searches for the optimum operating point and finds it at 864 V. This means that
all strings are operated at 864 V.
As a result, the short strings (blue curve) are operated at their ideal MPP and the red string, which
deviates due to its configuration, is operated slightly outside its MPP. Operating the long string outside its
ideal operating point leads to mismatch losses for this individual string. However, this does not lead to
any power reduction for the other strings.
Figure 2: Optimum operating point for strings of different lengths within a PV system
864
904
Loading...